Important parameters in precursor synthesis

As discussed in the preceding section, supported microporous membranes require precursor sols with weakly branched polymeric species with a fractal 

Important parameters are the concentration and concentration ratios of the components, the way the components are added to the final mixture, the temperature and ageing processes. Hydrolysis under strongly acidic conditions is necessary to obtain polymeric specimen. Characterisation is usually done by SAXS measurements. 

The synthesis conditions which lead to weakly branched systems involve the use of an acid catalyst where pH 2.2 (iso-electric point of silica) and the use of low to moderate water content (r 10). Hydrolysis (see reactions in Section 7.4) then takes place via a fast protonation of the alkoxide, followed by attack of water, resulting in the substitution of the alkoxy group with an hydroxyl group. Protonation becomes slower when more hydroxyls are present. The hydrolysis rate will therefore decrease with the extent of OH substitution. Acid catalysed condensation reactions proceed analogously where a protonated silanol species is attacked by water. The condensation reaction rate decreases with the number of condensed Si-O-Si groups. 

Condensation reactions under acid catalysed conditions are much slower than hydrolysis reactions and generally start when the hydrolysis process is almost complete. The largest differences in reaction rate constants for hydrolysis and condensation are reported for pH = 0.9 and these differences decrease if the pH is increased [56]. As a consequence a large amount of hydrolysed species is present at the moment condensation becomes significant. Further condensation reactions then take place between individual hydrolysed species (clusters) and lead to aggregated clusters. This is schematically represented for a simple case in Fig. 8.22 where dimers react with each other leading to a linear molecule. Further condensation reactions with other condensed polymers will take place preferentially at the end groups [54]. 

Cluster-cluster reactions are especially important during ageing. In general the hydrolysed species are not simple dimers but are weakly branched species 

---